1. Field of the Invention
The present invention relates to a noise filter array in which a plurality of filter elements including an LC parallel resonant circuit and an LC series resonant circuit, each of which includes a coil and a capacitor, are arranged in an array and integrally provided.
2. Description of the Related Art
Various communication modes (GSM mode, DCS mode, and PCS mode, for example) are used for cellular phones. To prevent degradation in the receiving sensitivity in respective communication bands used for these communication modes, it is necessary to effectively remove noise in each of the communication bands.
For example, to remove noise in respective communication bands around 900 MHz and around 1.8 GHz, a noise filter is required to obtain attenuation over a wide band range. To provide a filter having such a wide-range attenuation characteristic, an inductance may be provided by a grounded capacitor to form a double-resonance filter.
Then, in the related art, as shown in FIG. 13, a double-resonance filter element has been disclosed which includes an LC parallel resonant circuit PR arranged so that a stray capacitance C1 is provided in parallel with a coil L1 provided in a signal line and an LC series resonant circuit SR arranged so that a capacitor C2 and a coil L2 are connected in series between the signal line and a ground (for example, Japanese Unexamined Patent Application Publication No. 9-266430).
When a noise filter array is arranged so that a plurality of the existing double-resonance filter elements are provided, and these filter elements are integrally provided with one another, an array structure, for example, has an equivalent circuit as shown in FIG. 14. Note that here, a configuration in which four filter elements are provided is shown as an example.
In the array structure shown in FIG. 14, any LC series resonant circuit portions surrounded by the broken lines in the drawing are grounded. Thus, in order to further simplify the structure, coils included in the respective LC series resonant circuits are integrated. In this case, for example, the structure will be as shown in FIG. 15.
That is, in the noise filter array shown in FIG. 15, signal-side electrodes of capacitors c12 to C42 that define the LC series resonant circuits of the respective filter elements are provided separately for the respective filter elements, while a conductor L0 that defines an inductance coil is commonly connected to ground-side electrodes of these capacitors C12 to C42. Thus, the structure is simplified.
The noise filter array having the structure shown in FIG. 15 can simplify the overall structure. However, distances from the capacitors C12 to C42 of the filter elements F1 to F4 via the conductor L0 to a ground terminal GND are different among capacitors C12 to C42 (for example, the distance from the capacitor C12 to the ground terminal GND is different from the distance from the capacitor C22 to the ground terminal GND). Thus, an inductance value varies among the filter elements F1 to F4 and, as a result, variations in filter characteristic arise among the filter elements F1 to F4.